>>I see strange things.>>When I use bittorrent (azureus or bittorrent-gui), at the same time as>>iptables (for nat and internet access for my ibook), when I stop a download>>or exit from one of this apps my external network goes down. >>I have tried the same without iptables loaded and it works fine.

What exactly do you mean with "network goes down"? Can you find outwhere the packets disappear? Do they silently disappear, or do you getan error code from sendmsg? What about received packets?